Elemental sulfur has found considerable utility in agriculture as a soil amendment, particularly for high-clay, alkaline soils, and as a source of sulfur (in the form of sulfate) for plant growth. Two forms are generally used for direct soil application: aqueous sulfur suspensions, similar to the very common suspension fertilizers, and dry sulfur particles. Both require that the sulfur be in a particulate form. However, it is readily apparent that different particle sizes are required, since only very fine particles will form suspensions having desirable stability and flow properties, while considerably larger particles are desired to prevent dusting and other handling problems for dry application.
The more common sources of sulfur are the Frasch process mines, in which heated water or steam is injected into underground sulfur deposits to melt the sulfur, whereupon it is pumped to the surface, and the conversion of hydrogen sulfide, extracted from petroleum refinery process streams, into elemental sulfur. In either instance, the produced sulfur is usually made available in the molten state, and then is discharged into a storage area to solidify into large blocks.
Upon breaking the blocks, the sulfur can be ground into the required sizes (usually 100 to 200 mesh) for suspension production. This grinding, however, must be conducted with great care and under an inert atmosphere, due to the flammability of sulfur and the explosive tendencies of sulfur dust.
Although the fluid sulfur suspensions are a very convenient source, from the standpoint of application ease, they create serious storage problems. The finely divided sulfur which is used in the preparation of suspensions is itself difficult to store because of its tendency to dust or cake into large chunks. In addition, the previously noted flammability and explosion hazards remain as problems during storage.
These storage problems can be alleviated by converting the sulfur to aqueous suspensions, but the result is a rather inefficient utilization of expensive tankage, due to the fact that even the most concentrated sulfur suspensions commonly used (50 to 60 weight percent sulfur) contain 40 or more percent by weight of water. Because of this, the suspensions can require twice as much storage volume as would the dry sulfur particles. Furthermore, the air oxidation of sulfur gradually produces sulfuric acid, requiring that more expensive corrosion-resistant materials be used for long-term suspension storage.
To save shipping expenses, the sulfur suspensions are usually prepared at a fertilizer blending plant located in a rural area near the location where the suspensions are to be applied. These plants are typically quite small and equipped with only simple mixing equipment and some storage tanks. They are not customarily able to perform the technically difficult, hazardous operations such as grinding sulfur into fine particles. For this reason, sulfur for suspensions must be purchased in the required small particle size, resulting in a rather expensive suspension product.
U.S. Pat. No. 3,799,884 to Young, which is incorporated herein by reference, describes a sulfur suspension which can be prepared from particles of a special porous sulfur (as disclosed in U.S. Pat. No. 3,637,351 to Young et al.) having a size generally greater than 50 mesh. The preparation involves adding porous sulfur particles to water, contained within the storage tank of an apparatus used for application of a suspension to the soil or plants, and pumping the mixture in a closed circulation system through the pump of the applicator and back into the storage tank. After prolonged pump operation, the particles are reduced in size by grinding inside the pump housing to a point at which they can be applied. Advantages claimed for the invention include the ability to use a sulfur source more convenient than finely ground sulfur, and the elimination of suspension settling problems during transportation, since suspensions are produced at the site where they are to be used.
The process of Young, however, does not produce a suspension which is equivalent to those which are commercially used, due to the somewhat larger particle sizes which are measured after the practice of his invention (e.g., the obtaining of only about 12 percent by weight of particles which will pass through a 200 mesh sieve in Example 3 of the patent, even after prolonged grinding). These suspensions probably could not be easily re-dispersed after even a short period of shipment, so must always be prepared at the location of their ultimate use. In addition, the process suffers from the need for quite lengthy circulation times through the pump system to achieve a useful suspension. A further limitation of the process is its inability to utilize molten sulfur as a feed material, which is the least expensive form available in some areas.
Accordingly, it is an object of the present invention to provide sulfur suspensions containing substantial amounts of particles smaller than about 200 mesh without utilizing finely ground sulfur as a starting material.
It is a further object to permit the use of equipment normally present in fertilizer blending plants for preparing sulfur suspensions.
A still further object is the use of molten sulfur to prepare sulfur suspensions in a single vessel, using an uncomplicated process.
Another object is the use of molten sulfur to prepare a mixture, in the desired proportions, of sulfur suspension and non-dusting sulfur particles which can be stored for subsequent application to the soil or processing into sulfur suspension.
These, and other objects will more clearly appear from consideration of the following description and examples.